Download Laser oscillation in proton implanted Nd:YAG waveguides

Laser oscillation in proton implanted Nd:YAG waveguides
a
M. Domenech*a, E. Cantelara,G.V. Vázquezb and G. Lifantea
Depto. Física de Materiales, C-IV. Universidad Autónoma de Madrid
28049 – Madrid (SPAIN)
b
Centro de Investigaciones en Óptica, Loma del Bosque 115, Lomas del Campestre, Apartado Postal
1-948, 37000 León, GTO., (MÉXICO)
*
e-mail address: [email protected]
This work reports continuous laser oscillation at λ=1064.4 nm at room temperature in Nd:YAG
waveguides fabricated by proton implantation technique. The resulting index profiles after the multiimplant, the spectroscopic characteristics of the Nd3+ ions in the waveguide, as well as the laser
characteristics of the proton implanted Nd:YAG waveguide laser are reported.
Keywords: laser, waveguide, rare earths, ion implantation.
Introduction
Nd doped YAG is one of the most attractive dielectric materials for solid state lasers. The
efficiency of laser operation is enhanced by using waveguide geometries, because the optical
modes propagate well confined in the waveguide structure, avoiding thus diffraction effects.
Several techniques have been proposed to fabricate optical waveguides in Nd:YAG, including
epitaxially grown Nd:YAG layers on pure YAG substrates [1], or helium implantation on Nd
doped YAG crystals [2], allowing high slope efficiency and low threshold laser operation at 1.06
µm. In fact, Nd:YAG was the first material that demonstrated the suitability of the ion implantation
technique to fabricate waveguide lasers [3].
The ion implantation process produces at the end of the ion track an amorphization of the
crystal, giving rise to a decrease of the refractive index in many dielectric materials [4]. This lowdensity region generates an optical barrier that confines the radiation, producing an optical
waveguide. One of the problems faced when He+ is used to produce optical waveguides is the short
range of the implanted ions. Typically, a 2.8 MeV ion energy produces an optical barrier situated at
around 6 µm beneath the surface. As higher energies is difficult to achieve in practice, a different
approach becomes necessary. An alternative to fabricate wider waveguides using ion implantation
technique is to use proton instead of He+ ions, as for a given energy the ion range is much deeper in
the case of lighter ions [5].
In this work the characterization of a Nd:YAG waveguide laser operating at 1.06 µm
fabricated by proton implantation is reported. The characterization includes the waveguide index
profile induced by the ion implantation, the main spectroscopic features of the Neodymium ions
inside the waveguide, as well as the laser characteristics such as slope efficiency and threshold
obtained using a Ti:Sapphire as the pump source.
Experimental Procedure
A planar waveguide was fabricated at room temperature on Nd:YAG by the technique of ion
implantation using protons of energy around 1 MeV. In order to produce a broad barrier to avoid
tunneling losses, a multi-implant was performed in the Nd:YAG substrate. Four different implants,
with energies of 1.0, 1.05, 1.1 and 1.25 MeV, were performed on a single substrate, with a total
dose of 6x1016 ions/cm2 .
To obtain the waveguide refractive index profile, the propagation constants of the modes were
measured by the standard m-line method, using a rutile prism to couple the light into the waveguide
coming from a polarized He-Ne laser (λ=633nm).
A CW Ti:sapphire laser, with a tuning range between 750-850 nm, was used as the excitation
source. The pump beam was coupled into the waveguide with a x10 microscope objective by the
end-fire coupling technique. The output light was collected through a x20 microscope objective
and directed to the entrance slit of a monochromator (ARC SpectraPro 500-i) being detected by
using an InGaAs photodiode.
A laser cavity was formed by butting mirrors to the polished end-faces of the waveguide. A
>99.9 % reflectivity mirror at 1064 nm and transmission of 98% at 816 nm was placed in the front
face, while on the other face a 97% at 1064 nm and >99.8% at 816 nm reflecting mirror was used.
The pump power as well as the laser output power from the waveguide were measured by a silicon
detector (Newport Model 1815-C and Spectra Physics Model 407A power meters).
Results and discussion
Figure 1 shows the calculated refractive index profile of the resulting waveguide after the
multi-implant, using the experimental dark mode set measured at 633 nm. The profile exhibits an
optical barrier height of approximately 0.98 % (decrease in refractive index relative to the
substrate) located at a depth of 9.5 µm induced by the ion implantation process. Note that the
profile in the figure is plotted as an index decrease in order to emphasize the concept of “optical
well” and “optical barrier”. It is also important to remark that the effective index in the surface
region is slightly higher than that of the substrate by a 0.03%.
1,805
Proton implanted Nd:YAG
λ = 633 nm
Refractive index profile
1,810
Optical barrier
1,815
1,820
Light confinement region
Substrate
1,825
1,830
0
2
4
6
8
10
12
Depth (µm)
Figure 1: Refractive index profile corresponding to a proton implanted waveguide, measured at λ = 633 nm.
Before the laser experiments, the main spectroscopic characteristics of the Nd3+ ions in the
waveguide were studied, using a Ti:sapphire as excitation source. After excitation to the 2 H9/2 :4 F3/5
manifold (λ= 816 nm) the neodymium ions relax non- radiatively to the 4 F3/2 level. From this level
the relaxation is mainly radiative to the lower levels, giving rise to the apparition of three nearinfrared emission bands at around 940, 1064 and 1340 nm corresponding to 4 F3/2 → 4 I9/2 , 4F3/2 →
4
I11/2 , 4 F3/2 → 4 I15/2 transitions, respectively. The spontaneous spectrum associated to the radiative
relaxation from the 4 F3/2 level, measured in the waveguide, which exhibits the highest emission
cross section, is presented in figure 2. The luminescence from the waveguide shown in this figure
is coincident with that previously reported from bulk in Nd:YAG [5] having the same structure.
Also, the lifetime measured in waveguide configuration is coincident to that measured in bulk
crystal, around 240 µs.
0,6
Intensity (arb.units)
Intensity (arb.units)
0,6
0,5
0,4
0,5
0,4
0,3
0,2
0,1
0,0
1050
0,3
1060
1070
1080
Wavelength (nm)
0,2
0,1
0,0
1020
1040
1060
1080
1100
1120
1140
1160
Wavelength (nm)
Figure 2: Spontaneous emission of the Nd3+ ions in waveguide configuration after pumping at 816 nm. The
inset shows the laser output spectrum using 120 mW pump power.
It is well known that when neodymium is coupled to a resonant cavity it can operated as a four
level scheme leading to laser action [6]. By fabricating a laser cavity with two mirrors attached
directly to the proton implanted Nd:YAG waveguide, an intense infrared beam is observed. If
stimulated emission occurs, the 4 F3/2 → 4 I11/2 transition dominates over all other de-excitation
processes. The inset of figure 2 shows the recorded spectrum of the laser emission after pumping at
816 nm, where a narrow band centered at 1064.4 nm with a full width at half maximum (FWHM)
of 3 nm, is observed. This laser emission corresponds in fact to the maximum gain transition of the
Nd3+ ions in YAG crystals.
Output Power (mW)
4
Proton implanted
Nd:YAG
3
2
1
0
0
20
40
60
80
100
120
140 160
180
Pump Power (mW)
Figure 3: Output characteristics of the proton implanted waveguide Nd:YAG laser, showing a threshold of
around 54 mW pumping at 816 nm.
The output characteristics of the proton implanted Nd:YAG waveguide operating in CW mode
are given in figure 3, where the laser output power versus the pump power is presented. The pump
power needed to reach laser oscillation gives a threshold of Pth = 54 mW, being the slope efficiency
(the ratio of the output power to pump power above threshold) around 5%. The laser output showed
a very high stability, even under continuous wave pump operation at room temperature, which
clearly confirms the excellent mechanical, thermal and optical properties of the YAG matrix,
besides the suitability of the ion implantation to construct miniaturized integrated devices.
Acknowledgements
Work partially supported by Ministerio de Ciencia y Tecnología (Spain) under project
TIC2002-00147.
References
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(submitted for publication)
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